This invention relates generally to magnetic circuit protecting devices, and more particularly to reed switch apparatuses that include means to actuate and latch reed switches in response to a predetermined fault condition.
A reed switch is usually composed of two magnetizable and conductive flat metal reeds cantilevered inwardly toward each other from glass to metal bonds at the ends of a surrounding tubular glass capsule. The leads also extend axially outward from the bonds of the capsule as conductive leads. The capsule forms an airtight seal around the reeds. The flat reeds are cantilevered so that the flat surfaces on their interior ends face each other, but are separated by a gap.
Such a reed switch is actuated by an axial magnetic field which can be applied by a magnet or other means. This magnetizes the reeds enough so their interior ends are attracted and moved toward each other until contact is made. This type of switch is known as a Form-A reed switch.
Also available is a Form-C reed switch which utilizes three reed elements and performs a single-pole, double-throw switching function. The more common of these has a short non-magnetizable reed and also a short magnetizable reed sealed at one end of the glass capsule. A longer magnetizable reed, sealed in the opposite end has its flattened portion positioned between both reeds with on side resting against the face of the non-magnetizable reed, effecting a normally closed contact.
Therefore, whenever the reed switch changes from a normal status to an operated status, the longer, more flexible reed, breaks contact with the non-magnetizable reed makes contact with the magnetizable reed by moving laterally from the former to the latter reed.
Other versiosn of the Form-C reed switch wherein all reed elements are magnetizable, are also applicable to this invention. The mode of operation of one of these is disclosed in the specification of U.S. Pat. No. 3,229,063, issued on Dec. 11, 1963, to M. J. Sakatos.
The invention encompasses all types of reed switches in the versions to be depicted.
A characteristic of most Form-C reed switches which had probably precluded prior consideration for circuit protector use, is the tendency of the movable contact to hesitate, or hang-up, between the normally closed and normally open contacts at some level of flux when exposed to a slowly increasing magnetic field. Additional flux is required for the read to continue its travel to the opposite contact.
Prior to the invention, any attempt to utilize the normally closed contacts of a Form-C reed switch to interrupt the load current in response to the flux generated by a series connected sensing coil surrounding the switch would prove highly unsuccessful because as the magnetic field reached the critical level, it would suddenly collapse as the transfer reed contact, series connected to the coil, began to open The final result would likely be contact failure due to arcing as the movable reed "buzzed" open and closed.
Another characteristic of all reed switches is that after the reeds have been attracted to one another, a reduced magnetic field is sufficient to hold them in the actuated, or latched, position because the poles formed at the reed ends are now very close and exhibit a stronger attractive force than when they are apart. The field must be reduced to a significantly lower value before the switch drops out, i.e., before the magnetic attraction between the interior reed ends is insufficient to overcome the stress forces that drive the reeds to their rest position.
This characteristic makes it possible to provide a magnetic field, between the pull-in value and the drop-out value, in the form of a permanent magnet, or coil winding, which will latch the switch in the state of conduction into which it was last actuated. The advantage of this will become self evident as the invention is disclosed.
At present, the only alternative to fuses for overcurrent protection of electronic equipment has been the magnetic circuit breaker, but its effectiveness has been questionable. One of the major limitations is the slow reaction time between sensing and the subsequent response to the fault. The prime cause of this is that circuit breakers generally comprise a plurality of electrical and mechanical parts, such as hinges, springs, linkages, etc., all of which are called on to function to accelerate contact opening and then latch the contacts in the open position.
Those skilled in the art appreciate that the mass and inertia of the mechanisms introduce significant delay in operating time. It will also be apprciated that the increased friction due to wear of the actuating mechanism can lead to increased delays as well as instability.
The invention to be disclosed accomplishes this purpose in a novel manner without resorting to auxiliary mechanisms.